There are many ways to burn $KOIN. One way is to interact with the PoB contract via https://koinosblocks.com/address/159myq5YUhhoVWu3wsHKHiJYKPKGUrGiyv.
Connect your Kondor Wallet and enter the amount of $KOIN to burn.
Another way is to use Koinos CLI which can be found here: Releases · koinos/koinos-cli · GitHub
you will need to import the wallet with the $KOIN you want to burn and then register the pob contract, then call pob.burn: